function objseTis(obj) {
  
    var str = ''
    for (var key in obj) {
        str += `${key}=${obj[key]}&`
    }
    return str.slice(0, str.length-1)
     }
 function ajax(method, url, params, cd) {
    // 1.创建一个xhr对象
    var xhr = new XMLHttpRequest()
    //2.配置xhr对象
    xhr.open(method, method=='get'?url+"?"+objseTis(params):url)
    //3.用回调函数来接收返回的响应
    xhr.onreadystatechange = function () {
        if (xhr.readyState == 4) {
            cd(JSON.parse(xhr.responseText));
        }
    }
    //4.发送请求
    //先对post请求设置 content-type 请求头
    xhr.setRequestHeader('content-type','application/x-www-form-urlencoded')
    xhr.send(method=='get'?null:objseTis(params))
     }